The field of this invention relates to prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia and accommodating for presbyopia.
It is well known that anomalies in the shape of the eye can be the cause of visual disorders. Normal vision occurs when light passes through and is refracted by the cornea, the lens, and other portions of the eye, and converges at or near the retina.
In a myopic or near-sighted eye, the cornea is too steeply curved for the length of the eye. This curvature causes light rays to converge at a point before it reaches the retina. Distant objects, therefore, appear out-of-focus or blurry since the light rays are not in focus by the time they reach the retina. Approximately one in four persons have myopic vision.
In persons who are older, a condition called presbyopia occurs in which there is a diminished power of accommodation of the natural lens resulting from the loss of elasticity of the lens. Ordinarily the eye may vary its optical power by focusing the natural lens. However, with the loss of lens elasticity, the eye muscles cannot bend or focus the lens needed for clear vision of near objects. Typically presbyopia begins about the age of 40 and becomes significant after the age of 45.
Corrections for myopia and presbyopia have been attempted primarily through the use of prescriptive lenses in the form of glasses. Many adults wear bifocals or trifocals to correct their vision to see clearly at different distances. Generally, a bifocal lens is arranged such that the upper portion of the lens is used for distance vision and the lower portion for near vision. For reading, a person looks through the lower portion of the lens.
Additionally, available for the correction of myopia and presbyopia are hard, gas-permeable, and soft contact lenses. These contact lenses come in a variety of designs and provide bifocal correction. Also known for multi-vision correction are diffractive contact lenses where the surface of the lenses have invisible ridges molded into concentric circles. Generally, light passing through the lens is bent so that the wearer""s near and distant vision is corrected. Few persons of older age, however, are able to adjust to the use of contact lenses. This is especially true as many older persons have trouble inserting and removing the contacts on a daily basis from their eyes.
Also, correction of myopia through the use of various corneal implants within the body of the cornea have been suggested. Various designs for such implants include solid and split-ring shaped, circular flexible body members and other types of ring-shaped devices that are adjustable. These implants are inserted within the body of the cornea for changing the shape of the cornea, thereby altering the its refractive power. Although these corneal implants attempt to correct for the myopic condition, they do not adequately accommodate for presbyopia.
The present invention is directed to a bio-compatible corneal implant for correction of myopia and accommodation for presbyopia. The corneal implant is ring-shaped and made from a bio-compatible material with a lens body having an inner and outer circular edge. The inner circular edge forms an opening in the lens body. The posterior surface of the lens body has a uniform radii of curvature between the inner and outer circular edges. The anterior surface has two radii of curvatures. The first radii of curvature extends from near the outer circular edge to a junction point before the inner circular edge. The second radii of curvature extends from the junction point and continues to the inner circular edge. In one embodiment, an aspherical surface transitions the first radii to the second radii of curvature. This aspherical curvature provides a smooth transition between each radii of curvature which reduces the thickness of the junction point. The inner and outer circular edges have a thickness less than about 0.020 mm and preferably about 0.010 mm.
The corneal implant, when placed under a lamellar dissection made in the cornea (such as a corneal flap), to relieve tension of Bowman""s membrane, alters the outer surface of the cornea to correct the refractive error of the eye. By relieving the pressure and subsequent implantation of the device, the pressure points which typically are generated in present corneal surgeries are eliminated, and hence reduces risk to patients of extrusion of implants. Unlike an implant placed in an intact cornea where corneal tissue can be deflected anteriorly or posteriorly leading to unpredictable refractive correction, proper relief of the pressure due to dissection of the Bowman""s membrane ensures that all the corneal changes take place at the anterior surface.
For the correction of myopia, the implant is shaped into a meniscus lens with an anterior surface of the second radii of curvature being flatter than the posterior surface. When the implant is placed concentrically on the stromal bed the curvature of the anterior surface of the cornea in the optic zone is flattened to the extent appropriate to achieve the desired refractive correction.
For the accommodation of presbyopia, the corneal implant has a circular hole. When implanted on the stromal bed and the corneal flap is positioned over the anterior surface of the corneal implant, a slight anterior oriented curvature is thereby retained in the center of the cornea leaving extra power needed for near vision, thus correcting for presbyopia.
The material from which the corneal implants are made is preferably a clear, permeably, microporous hydrogel with a water content greater than 40% up to approximately 90%. The refractive index should be substantially identical to the refractive index of corneal tissue. Other bio-compatible materials from which the corneal implant may be made, include: polymethlmethacrylate (PMMA), silicone polymers, UV-absorbing acrylic, hydrogel, microporous hydrogel, collamer, collagel acrylic polymers, and other composite materials.
The refractive index of the implant material should be in the range of 1.36-1.39, which is substantially similar to that of the cornea (1.376). This substantially similar refractive index prevents optical aberrations due to edge effects at the cornea-implant interface.